Reversible periodic magnetic focusing system

ABSTRACT

The reversible periodic magnetic focusing system according to the invention comprises a successive arrangement of permanent magnets 7 magnetized in the opposite sense, and pole shoes 1 interposed between the permanent magnets 7. Each pole shoe 1 is provided with a hole for passage of an electron flow. The holes of the pole shoes 1 receive grids 2 with meshes 3. The grids 2 are of a magnetically soft material and have a magnetic, thermal and electric contact with the pole shoes 1. Respective meshes 3 of the grids 2 are arranged coaxially. 
     The invention stipulates ratios between the geometrical dimensions of components of the reversible periodic magnetic focusing system, which ensure passage of electron beams 5 through all the meshes 3 of the grids 2. 
     The invention is applicable to the electronic industry where it can be used to design and manufacture, compact, low-voltage, superhigh frequency, high-power devices, such as klystrons and travelling wave tubes. The invention is also applicable to charged particle accelerators and equipment which makes use of extended electron flows.

FIELD OF THE INVENTION

The present invention relates to vacuum tubes and, more particularly, toa reversible periodic magnetic focusing system.

REVIEW OF THE PRIOR ART

Superhigh frequency tubes, such as klystrons and travelling wave tubesfind extensive application in different branches of the economy in manycountries. The factors that hinder their application are their greatweight and the necessity of using high voltage sources. The weight ofsuch tubes and their anode voltage are largely determined by the weightof the focusing system and the perveance of the electron flow.Light-weight focusing systems capable of forming high-perveance electronflows are an indispensable condition for the provision of light, lowvoltage superhigh frequency tubes.

At present use is made of different types of focusing systems.

There are known magnetic focusing systems (cf. U.S. Pat. No. 3,475,644,Cl. 315-35, of Oct. 28, 1969) of the type that comprises a solenoid withpole shoes mounted on its end faces. As the solenoid is excited, auniform magnetic field is produced between the pole shoes, whichprovides for the transportation of a high-perveance flow exiting from agun.

Such focusing systems are disadvantageous in that they have a greatweight and size and require a special power source.

There are known focusing systems which use a permanent magnet instead ofa solenoid to produce a uniform magnetic field over the entire length ofthe system (cf. G. Merdinian and J. V. Lebacqs, "High Power, PermanentMagnet Focused, S-Band Klystron for Linear Accelerator Use" in Proc. 5thInt. Conf. on Hyperfrequency Tubes, Paris, France, Sept. 1964).

Such systems require no special power source, but suffer from a greatweight and size.

There are known electrostatic focusing systems (cf. U.S. Pat. No.3,436,588, Cl. 3155.39, of Apr. 1, 1969) comprising a plurality ofsingle electrostatic lenses arranged between the resonators of theklystron. Such systems have a small weight and size and require nospecial power source.

However, electrostatic systems do not provide for a sufficiently strongfocusing; the perveance of the electron flow focused by such systems isnever higher than 1·10⁻⁶ A/B^(3/2).

There is known a reversible periodic magnetic focusing system (cf. FRGPat. No. 1,190,708, Cl. 21 g 13/17, of December 1965) comprising aplurality of successively arranged pole shoes with a central hole forthe passage of an electron flow. Permanent magnets are used to produce auniform variable-polarity (reversible) magnetic field between the poleshoes.

The focusing of such a system is stronger than that of an electrostaticsystem; the latter system is also advantageous in that it makes itpossible to reduce the weight of the magnet by (n+1) times, as comparedto the system with a uniform magnetic field, where n is the number offield reversals.

However, reversible magnetic systems can only operate at a smallperveance of the electron flow. This is due to the presence of areversal zone in which the magnetic field is weak. The extension of thiszone is commensurable with the diameter of the hole provided in the poleshoe. As a high-perveance electron flow passes through the reversalzone, it gets out of focus and its further formation is disturbed.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the present invention to increase the perveance ofthe electron flow formed by a reversible periodic magnetic focusingsystem.

The invention essentially aims at providing a reversible periodicmagnetic focusing system which would reduce the extension of thereversal zone by several tens of times and would ensure thetransportation of a high-perveance electron flow due to an improved poleshoe design.

The foregoing object is attained by providing a reversible periodicmagnetic focusing system comprising a successive array of permanentmagnets magnetized in the opposite sense, and magnetically soft poleshoes interposed between the permanent magnets, each pole shoe having ahole for the passage of an electron flow, the system beingcharacterized, in accordance with the invention, in that the holes ofthe pole shoes receive grids with meshes, the grids being of amagnetically soft material and having a magnetic, thermal and electriccontact with the pole shoes, the respective meshes of the grids of thepole shoes being arranged coaxially.

The periodic magnetic focusing system according to the invention reducesthe extension of the reversal zone of the magnetic field several tens oftimes and proportionately increases the perveance of the electron flowformed by the system.

It is advisable that the pole shoes and meshes of the grids should beround, and that the flat pole shoes should be provided on each side withannular projections with radially magnetized ring magnets attachedthereto, the dimensions and thickness of the grids, the distance betweenthe pole shoes, the diameter of the meshes of the grids, the diameter ofthe pole shoes and the size of the projections being selected so as tomeet the following conditions:

    0.8≦(a/t)≦1.3                                (1)

    1≧(3/4)R(B/B.sub.1);                                (2)

    0.05≦(h/L)≦0.15;                             (3)

    (d/L)≦0.6                                           (4)

    0.8≦(D/L).                                          (5)

where

a is the diameter of the meshes of the grids;

t is the grid thickness;

l is the aximuthal distance between the nearest holes which areequidistantly spaced from the centre of the pole shoes;

R is the distance between the centre of a mesh of a grid and the centreof the pole shoe;

B is the induction in the gap between the pole shoes;

B₁ is the maximum induction of the linear portion of the magnetizationcurve of the pole shoe material;

h is the height of the projections;

L is the distance between the pole shoes;

D is the outer diameter of the pole shoes.

Meeting the above conditions ensures a uniform magnetic field in eachmesh of the grid and throughout the spacing between the pole shoes.This, in turn, provides conditions for the correct formation of theelectron flow by all the meshes of the grids.

Each pole shoe can be shaped as a polyhedron with parallelepiped-shaped,longitudinally magnetized permanent magnets arranged radially on thefaces of the polyhedron.

Such a design considerably facilitates the magnet manufacture.

It is expedient that the grids of the pole shoes should have a honeycombstructure with hexagonal meshes.

This makes it possible to improve the transparence of the grids andraise the perveance of the electron flow focused by the system.

The pole shoes may be shaped as rectangular plates, in which case twoaxially magnetized permanent magnets are arranged one opposite the otherbetween the pole shoes, the permanent magnets arranged in the gapbetween pole shoes being displaced in the azimuthal direction by 90°with respect to the permanent magnets arranged in the next gap.

This intensifies the magnetic field and improves its uniformity; such asarrangement also facilitates the location of the power input and outputmeans and provides a ready access to the means for adjusting theresonator of the klystron and to the resonator unit cooling system.

The pole shoes may be constructed as cross-shaped plates, in which casefour axially magnetized, prism-shaped permanent magnets are arrangedbetween the ends of the cross, the magnets having a polyhedralcross-section.

This shape of the permanent magnets provides enough space for thelocation of the power input and output means and for a ready access tothe resonator adjustment means.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 is a schematic view of a reversible periodic magnetic focusingsystem in accordance with the invention;

FIG. 2 is a view of a pole shoe with round meshes of the grid;

FIG. 3 is a view of a pole shoe with a polyhedron-shaped lateralsurface;

FIG. 4 is a view of the pole shoe of FIG. 3 with hexagonal meshes of thegrid;

FIG. 5 is a schematic view of a reversible periodic magnetic focusingsystem in accordance with the invention with rectangular pole shoes;

FIG. 6 is a schematic view of a reversible periodic magnetic focusingsystem in accordance with the invention with cross-shaped pole shoes;

FIG. 7 is a schematic diagram of a klystron with a reversible periodicmagnetic focusing system in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reversible periodic magnetic focusing system according to theinvention comprises pole shoes 1 (FIGS. 1 and 2) into which there areinstalled grids 2 with meshes 3 arranged coaxially with the outlet holesof a multibeam electron gun 4. Electron beams 5 of the multibeamelectron flow of the gun 4 pass through the meshes 3. On each side, thepole shoes 1 are provided with annular projections 6 on which there aremounted ring permanent magnets 7 magnetized in the radial direction andproducing a reversible magnetic field. The permanent magnets 7 areinterconnected by a magnetic circuit 8 of a magnetically soft material.

As the grids 2 are introduced into the holes of the pole shoes 1, allthe meshes 3 of the grids 2, with the exception of the central mesh, aredisplaced with respect to the geometrical centres of the pole shoes 1.This affects the axial symmetry of the magnetic field in the displacedmeshes 3 and impairs the focusing of the electron beams 5.

The lack of symmetry of the magnetic field can be ruled out by providinga uniform magnetic field in the gaps between the grids 2 and by makingthe radial component of the magnetic field independent of the azimuthaldisplacement angle of each mesh 3 of the grids 2.

The uniformity and distribution of the magnetic field in the gapsbetween the grids 2 are determined by the ratio between the height ofthe projections 6, the diameter of the pole shoes 1 and the distancebetween adjacent pole shoes.

With an increased height of the projections 6, the induction of themagnetic field increases towards the middle of the gap between the poleshoes 1, and the lines of force of the magnetic field are saddle-shaped.With a small height of the projections 6, the induction of the magneticfield decreases towards the middle of the gap between the pole shoes 1,and the lines of force of the magnetic field are barrel-shaped. Auniform magnetic field can only be produced between the grids 2 with anoptimum height of the projections 6, which can be determined throughcomputerized calculation of the magnetic system.

The uniformity of the magnetic field between adjacent pole shoes 1 isalso dependent upon the diameter of the pole shoes 1 and suffers from adecrease of that diameter. After the diameter of the pole shoes 1 isreduced to a certain value, the uniformity of the magnetic field can nolonger be improved by adjusting the height of the projections 6. Forthis reason, the diameter of the pole shoes 1 must be greater than acertain threshold value.

The magnetic field between the grids 2 becomes less uniform towards theperipheral meshes 3, for which reason the diameter of the grids 2 mustbe less than a certain threshold value.

An optimum height of the projections 6 and threshold values of thediameter of the pole shoes 1 and the diameter of the grids 2 can beestablished experimentally or through computer-aided calculation.Experiments and calculations indicate that in order to obtain a uniformmagnetic field in the gaps between the grids 2, the diameter D of thepole shoes 1, the diameter d of the grids 2, the distance L betweenadjacent pole shoes 1, and the height h of the projection 6 should beselected so as to meet the following conditions:

    0.05≦(h/L)≦0.15,

    0.8≦(D/L),

    (d/L)≦0.6.

The radial component of the magnetic field can be made independent ofthe azimuthal displacement angle of each mesh 3 of the grids 2 by acorrect selection of the aximuthal distance between the neighboringmeshes 3 which are equidistantly spaced from the centre of the poleshoes 1, and by correctly selecting the thickness of the grids 2.

The azimuthal distance between the neighboring meshes 3 of the grids 2determines the location of the operating point on the magnetizationcurve of the magnetically soft material of the grids 2. With a greatazimuthal distance between neighboring meshes 3 of the grids 2, theoperating point is on the linear portion of the magnetization curve ofthe material of the grids 2; in this case there is no saturation in thegrids 2, and the radial component of the magnetic field is independentof the azimuthal displacement angle of the meshes 3 of the grids 2. Asthe azimuthal distance between adjacent meshes 3 of the grids 2decreases, the operating point first moves to the non-linear portion ofthe curve and then to the portion corresponding to saturation of thematerial of the grids 2. In this case the radial component of themagnetic field is dependent on the azimuthal displacement angle of themeshes 3 of the grids 2. Thus in order to make the radial component ofthe magnetic field independent of the azimuthal displacement angle ofeach mesh 3 of the grids 2, the azimuthal distance between adjacentmeshes 3 of the grids 2 must be greater than a certain threshold value.

The necessity to equalize the magnetic fluxes passing through the grids2 and the gaps between adjacent pole shoes 1 suggests that the azimuthaldistance 1 between adjacent meshes 3 of the grids 2, which areequidistantly, spaced from the centre of the pole shoes 1, should beselected so as to meet this condition:

    1≧(3/4)R(B/B.sub.1),

where

R is the distance between the centre of the mesh 3 of the grid 2 and thecentre of the pole shoe 1;

B is the induction of the magnetic field produced by the magnets 7 inthe gaps between adjacent pole shoes 1;

B₁ is the maximum induction of the linear portion of the magnetizationcurve of the material of the grids 2.

The thickness of the grids 2 also influences the dependence of theradial component of the magnetic field upon the azimuthal displacementangle of the meshes 3 of the grids 2. With thin grids 2, the material ofthe grids 2 is saturated to impair the focusing of the electron beams 5.If the thickness of the grids 2 is substantially greater than thediameter of the meshes 3, the reversal zone of the magnetic field is tooextended in the meshes 3, which also impairs the focusing of theelectron beams 5. In the optimum case, the diameter of the meshes 3 mustbe close to the thickness of the grids 2 and must meet this condition:

    0.8≦(a/t)≦1.3,

where

a is the diameter of the meshes 3;

t is the thickness of the grids 2.

In order to facilitate the fabrication of the reversible periodicmagnetic focusing system, the radially magnetized ring permanent magnetsmay be replaced by longitudinally magnetized permanent magnets 9 (FIG.3) shaped as parallelepipeds. In this case a pole shoe 10 is shaped as apolyhedron with the permanent magnets 9 mounted on its faces. It must bepointed out, however, that a replacement of the radially magnetized ringmagnets 7 (FIGS. 1 and 2) by the longitudinally magnetized permanentmagnets 9 (FIG. 3) reduces the induction of the magnetic field in thegaps between adjacent pole shoes by 10 to 20 percent.

The perveance of the electron flow formed by the reversible periodicmagnetic focusing system is dependent upon the number of the meshes 3(FIGS. 1 and 2) in the grids 2; this number depends, in turn, upon themesh packing density. In order to increase this density, the grids 2with round meshes 3 are replaced by honeycomb-structured grids 11 (FIG.4) with meshes 12 shaped as regular hexagons.

As follows from the above ratios, the thickness of the projections 6(FIGS. 1 and 2) and the thickness of the radially magnetized permanentring magnets decrease with a decreasing spacing between adjacent poleshoes 1. This reduces the induction of the reversible magnetic fieldproduced between the pole shoes 1. With a spacing between adjacent poleshoes 1 less than 30 to 40 mm and with the necessity to produce ahigh-induction reversible magnetic field, it is expedient that theradially magnetized permanent ring magnets 7 should be replaced byaxially magnetized permanent magnets 13 (FIG. 5) shaped asparallelepipeds and interposed between rectangular pole shoes 14 (FIG.5). In order to improve the uniformity of the reversible magnetic field,the permanent magnets arranged in the gap between pole shoes must bedisplaced in the aximuthal direction by 90° with respect to thepermanent magnets 13 arranged in the next gap.

In order to further increase the induction of the reversible magneticfield, the pole shoes are constructed as cross-shaped plates 15 (FIG.6); four axially magnetized prism-shaped permanent magnets 16 arearranged between the ends of the cross. With the cross-section of themagnets shaped as a polyhedron, enough space is provided betweenadjacent permanent magnets 16 to locate power input and output means andprovide a ready access to the resonator adjustment means.

FIG. 7 refers to a reversible periodic magnetic focusing systemaccording to the invention incorporated in a klystron. The klystroncomprises an array of resonators 17 with flight-path tubes 18, having onone side the electron gun 4 and on the other side a collector 19.Interposed between the resonators 17 are the pole shoes 1 in which thereare installed the grids 2 with the meshes 3, The latter are coaxial withthe outlet holes of the electron gun 4 so that the electron beams 5 ofthe multi-beam electron flow produced by the gun 4 pass through themeshes 3. The radially magnetized permanent ring magnets 7 are mountedon the annular projections 6 of the pole shoes 1. The permanent magnets7 are interconnected by the magnetic circuit 8.

The reversible periodic magnetic focusing system according to theinvention operates as follows. The permanent ring magnets 7 produce amagnetic flux whose lines of force extend through the magnetic circuit8, the pole shoes 1 and the grids 2 and produce a magnetic field in theflight-path tubes 18 of the resonators 17 arranged in the gaps betweenthe pole shoes 1.

The adjacent pole shoes 1 are magnetized in the opposite sense, so thatthe magnetic field produced in the gaps between the pole shoes 1 isreversible.

If the diameter of the pole shoes 1, the spacing between the pole shoes1, the height of the projections 6, the thickness of the grids 2 and thediameter of the meshes 3 are selected according to the above-mentionedratios, the distribution of the magnetic field in each flight-path tube18 is symmetric in relation to the symmetry axis of the tube 18. Thediameter of the meshes 3 is relatively small; hence the extension of thereversal zone of the magnetic field is also limited.

Each electron beam 5 of the multibeam flow produced by the electron gun4 is introduced into the axially symmetric reversible magnetic fieldwith a limited reversal zone. If the perveance of each electron beam isrelatively small (for example, 0.5·10⁻⁶ A/B^(3/2)), the reversal zone ofthe magnetic field is too small to produce a significant defocusing ofthe electron beams 5 of the multibeam electron flow. As a result, theelectron beams 5 are in no way affected by the reversal zone of themagnetic field and reach the collector 19 of the klystron unimpeded.With several tens of electron beams 5 (for example, with 50 beams) thetotal perveance of the electron flow being focused is about 25·10⁻⁶A/B^(3/2).

To summarize, the reversible periodic magnetic focusing system accordingto the invention and the above-mentioned ratios between the dimensionsof its components make it possible to increase the perveance of theelectron flow to more than 25·10⁻⁶ A/B^(3/2), unlike conventionalperiodic magnetic focusing systems in which the perveance of the flow isabout 1·10⁻⁶ A/B^(3/2).

COMMERCIAL APPLICABILITY

The invention can be used to design and manufacture compact, high-powerUHF equipment, such as klystrons and travelling wave tubes. It is alsoapplicable to charged particle accelerators and to various equipment,such as welding and melting equipment, which makes use of extendedelectron flows.

What is claimed is:
 1. A reversible periodic magnetic focusing system to increase the perveance of the electron flow comprising:a successive arrangement of permanent magnets magnetized in an opposite sense; magnetically soft pole shoes interposed between the permanent magnets, each pole shoe having a hole for passage of an electron flow; and grids with meshes, receives in the holes of said pole shoes, said grids being of a magnetically soft material and having magnetic, thermal and electric contact with the pole shoes, the respective meshes of the grids of the pole shoes being arranged coaxially.
 2. A reversible periodic magnetic focusing system as claimed in claim 1, whereinthe pole shoes and the meshes are round, and annular projections on each side of said pole shoes, radially magnetized permanent ring magnets attached to said projections, and wherein the size and the thickness of the grids the distance between the pole shoes, the diameter of the meshes of the grids, the diameter of the pole shoes, and the dimensions of the projections are selected so as to meet the following conditions:

    0.8≦(a/t)≦1.3;

    1≧(3/4)R(B/B.sub.1);

    0.05≦(h/L)≦0.15;

    (d/L)≦0.6;

    0.8≦(D/L),

where a is the diameter of the meshes of the grids; is the thickness of the grids; l is the azimuthal distance between adjacent meshes which are spaced equidistantly from the centre of the pole shoes; R is the distance between the centre of the mesh of the grid and the centre of the pole shoe; B is the induction in the gap between the pole shoes; B₁ is the maximum induction of the linear portion of the magnetization curve of the material of the pole shoes; h is the height of the projections; L is the distance between the pole shoes; D is the outer diameter of the pole shoes.
 3. A reversible periodic magnetic focusing system as claimed in claim 1, wherein each pole shoe is shaped as a polyhedron on whose faces there are radially mounted longitudinally magnetized permanent magnets shaped as parallelepipeds.
 4. A reversible periodic magnetic focusing system as claimed in claim 1, wherein the grids of the pole shoes are honeycomb-structured grids with hexagon-shaped meshes.
 5. A reversible periodic magnetic focusing system as claimed in claim 1, wherein the pole shoes are constructed as rectangular plates with two axially magnetized permanent magnets arranged between the plates, the permanent magnets being displaced in the azimuthal direction by 90° with respect to permanent magnets arranged in the next gap.
 6. A reversible periodic magnetic focusing system as claimed in claim 1, wherein the pole shoes are constructed as cross-shaped permanent magnets with a polyhedral cross-section arranged between the ends of the cross.
 7. A reversible periodic magnetic focusing system comprising:a plurality of permanent magnets arranged successively and magnetized in the opposite sense; a plurality of magnetically soft pole shoes, each pole shoe being interposed between a pair of permanent magnets, and each pole shoe having a hole therein for passage of an electron flow; a plurality of grids constituted of magnetically soft material, said grids being receivable in said pole shoes such that they are in magnetic, thermal and electric contact with said pole shoes; and a plurality of meshes, a pre-determined number of said meshes being associated with each grid, the respective meshes of the grids being arranged coaxially.
 8. The reversible periodic magnetic focusing system of claim 7, and additionally comprising annular projections positioned on each side of the pole shoes, and magnetized permanent ring magnets attached to the annular projections, and wherein the pole shoes, meshes, and grids are round and the size and thickness of the grids, the distance between the pole shoes, the diameter of the meshes of the grids, the diameter of the pole shoes, and the dimensions of the projections are all selected so as to meet certain pre-determined conditions.
 9. The reversible periodic magnetic system of claim 8 wherein said pre-determined conditions are:

    0.8≦(a/t)≦1.3;

    1≧(3/4)R(B/B.sub.1);

    0.05≦(h/L)≦0.15;

    (d/L)≦0.6;

    0.8≦(D/L),

where a is the diameter of the meshes of the grids; t is the thickness of the grids; l is the aximuthal distance between adjacent meshes which are spaced equidistantly from the centre of the pole shoes; R is the distance between the centre of the mesh of the grid and the centre of the pole shoe; B is the induction in the gap between the pole shoes; B₁ is the maximum induction of the linear portion of the magnetization curve of the material of the pole shoes; h is the height of the projections; L is the distance between the pole shoes; D is the outer diameter of the pole shoes.
 10. The reversible periodic magnetic focusing system of claim 7 wherein said meshes include a central mesh with the others displaced relative to the geometrical centers of said pole shoes.
 11. The reversible periodic magnetic focusing system of claim 7 wherein the grids are honeycomb-structured and the meshes are hexagon-shaped.
 12. The reversible periodic magnetic focusing system of claim 7 wherein the pole shoes are rectangular plates having two axially magnetized permanent magnets arranged therebetween, said permanent magnets being displaced in the aximuthal direction by 90° with respect to permanent magnets arranged in the next gap.
 13. The reversible periodic magnetic focusing system of claim 7 wherein the pole shoes are cross-shaped plates, and wherein four axially magnetized prism-shaped magnets having a polyhedral cross section are arranged between the ends of the cross.
 14. The reversible periodic magnetic system of claim 9 wherein said meshes include a central mesh with the others displaced relative to the geometrical centers of said pole shoes.
 15. The reversible periodic magnetic focusing system of claim 9 wherein the grids are honeycomb-structured and the meshes are hexagon-shaped.
 16. The reversible periodic magnetic focusing system of claim 9 wherein the pole shoes are rectangular plates having two axially magnetized permanent magnets arranged therebetween, said permanent magnets being displaced in the azimuthal direction by 90° with respect to permanent magnets arranged in the next gap.
 17. The reversible periodic magnetic focusing system of claim 9 wherein the pole shoes are cross-shaped plates, and wherein four axially magnetized prism-shaped magnets having a polyhedral cross section are arranged between the ends of the cross. 